I. What is a switching power supply?
A switching power supply is a type of power supply that uses modern power electronics technology to control the on and off time ratio of a switching transistor to maintain a stable output voltage. Switching power supplies are generally composed of a pulse width modulation (PWM) control IC and a MOSFET.
A switching power supply is the opposite of a linear power supply. Its input directly rectifies AC power into DC power, and then, under the action of a high-frequency oscillation circuit, a switching transistor controls the current flow, generating a high-frequency pulse current. With the help of an inductor (high-frequency transformer), it outputs a stable low-voltage DC power.
Because the size of a transformer core is inversely proportional to the square of the operating frequency of the switching power supply, the higher the frequency, the smaller the core. This allows for a significant reduction in transformer size, resulting in a lighter and smaller power supply. Furthermore, since it directly controls DC, this type of power supply is much more efficient than linear power supplies. This saves energy, making it popular. However, it also has disadvantages: complex circuitry, difficult maintenance, and significant circuit pollution. The power supply also generates considerable noise, making it unsuitable for certain low-noise circuits.
II. Characteristics of Switching Power Supplies
Switching power supplies are generally composed of a pulse width modulation (PWM) control IC and MOSFETs. With the development and innovation of power electronics technology, switching power supplies are now widely used in almost all electronic devices due to their small size, light weight, and high efficiency, demonstrating their importance.
III. Classification of Switching Power Supplies
Based on how the switching devices are connected in the circuit, switching power supplies can generally be divided into three main categories: series switching power supplies, parallel switching power supplies, and transformer-type switching power supplies.
Transformer-type switching power supplies can be further divided into push-pull, half-bridge, and full-bridge types. Based on the transformer's excitation and the phase of the output voltage, they can also be classified as forward, flyback, single-ended, and dual-ended types.
IV. Differences between switching power supplies and ordinary power supplies
Ordinary power supplies are generally linear power supplies, meaning the regulating transistor operates in a linear state. Switching power supplies, however, are different. The switching transistor (in switching power supplies, we generally call the regulating transistor the switching transistor) operates in two states: on – very low resistance, off – very high resistance.
Switching power supplies are a relatively new type of power supply. They have advantages such as high efficiency, light weight, the ability to step up or step down voltage, and high output power. However, because the circuit operates in a switching state, the noise level is relatively high.
V. Example: Step-down switching power supply
Let's briefly explain the working principle of a step-down switching power supply: The circuit consists of a switch (in actual circuits, a transistor or MOSFET), a freewheeling diode, an energy storage inductor, and a filter capacitor.
When the switch is closed, the power supply provides power to the load through the switch and inductor, and stores some electrical energy in the inductor and capacitor. Due to the self-inductance of the inductor, the current increases relatively slowly after the switch is turned on, meaning the output cannot immediately reach the power supply voltage.
After a certain period of time, the switch is turned off. Due to the self-inductance of the inductor (which can be visualized as the current in the inductor having inertia), the current in the circuit will remain constant, continuing to flow from left to right. This current flows through the load, returns from the ground wire, flows to the positive terminal of the freewheeling diode, passes through the diode, and returns to the left end of the inductor, thus forming a loop.
The output voltage can be controlled by adjusting the opening and closing times of the switch (i.e., PWM – Pulse Width Modulation). If the opening and closing times are controlled by detecting the output voltage to maintain a constant output voltage, voltage regulation is achieved.
Both conventional power supplies and switching power supplies have voltage regulator transistors, using feedback principles for voltage regulation. The difference lies in that switching power supplies use a switching transistor for regulation, while conventional power supplies typically use the linear amplification region of a transistor. Comparatively, switching power supplies have lower power consumption, a wider range of AC voltage applications, and better DC output ripple. Their disadvantage is the presence of switching pulse interference.
The main working principle of a typical half-bridge switching power supply is that the upper and lower bridge switching transistors (VMOS at high frequencies) are turned on alternately. First, current flows in through the upper bridge switching transistor, and the energy is stored in the inductor coil using its energy storage function. Then, the upper bridge switching transistor is turned off, and the lower bridge switching transistor is turned on, allowing the inductor and capacitor to continuously supply power to the external circuit. Then, the lower bridge switching transistor is turned off again, and the upper bridge switching transistor is turned on to allow current to flow in, repeating this process. Because the two switching transistors are switched alternately, it is called a switching power supply.
Linear power supplies are different. Because there's no switch involved, the supply pipe is constantly releasing water. If there's excess, it leaks out. This is why we often see some linear power supplies generating a lot of heat in the regulating transistor; unused electrical energy is entirely converted into heat. From this perspective, the conversion efficiency of linear power supplies is very low, and high heat levels inevitably reduce component lifespan, affecting the final performance.
VI. Main differences: Working methods
In a linear power supply, the power regulator always operates in the amplification region, and the current flowing through it is continuous. Because the regulator loses a lot of power, a large power regulator is required, along with a large heatsink, resulting in severe heat generation and very low efficiency, typically between 40% and 60% (even with a very good linear power supply).
The operation of linear power supplies requires a voltage reduction device to convert high voltage to low voltage, typically a transformer, though other types like KX power supplies also exist. The output DC voltage is then rectified. This results in a large, bulky, and inefficient product with high heat generation. However, it also has advantages: low ripple, good regulation, low external interference, and suitability for analog circuits and various amplifiers.
A switching power supply operates its power devices in a switching state. During voltage regulation, energy is temporarily stored through inductors, resulting in low losses, high efficiency, and low heat dissipation requirements. However, this places higher demands on the transformer and energy storage inductor, requiring the use of low-loss, high-permeability materials. Its transformer is remarkably small. The overall efficiency ranges from 80% to 98%. While switching power supplies offer high efficiency and small size, their ripple and voltage/current regulation are somewhat compromised compared to linear power supplies.
